The present invention relates to a matrix print head which includes print needles propelled towards a print medium by means of electromagnetic drivers whereby in dependence upon the selection of the drivers, characters are generated out of a plurality of dots as the print head passes transversely to the column arrangement of the front ends of the needles. It is assumed moreover that the needles are mounted in a housing and are guided therein, and that the drivers are mounted in the rear portion of the housing. These drivers include pivotable armatures and yokes mounted to a base plate which in turn is connected to a rear portion of the housing, the latter in turn is connected to (or integral with) the front housing part that contains and guides the needles.
Matrix print heads of the type to which the invention pertains have armatures which are pivotably mounted in the housing such as an armature abuts against a yoke arm associated with and being part of the respective driver. The armatures are arranged in a kind of star pattern, and the radial inner part of each armature is, on one hand, connected to (or engages) the respective print needle during a forward stroke of that needle, while in a return position on the armature abuts against an annular disk being axially positioned with respect to a mounting sleeve of the rear housing part.
Matrix print head of the type and variety mentioned above include particularly flat rings made of a resilient material serving as abutment disk for the armatures. The rings are resilient for purposes of attenuating vibrations as well as undesired rebounding of the armatures on their return. This feature is shown in U.S. Pat. No. 4,230,412. Such resilient flat rings are disadvantaged by the fact that they vary the air gap between the other end of the armature and the front end of the respective magnetic core. This air gap must have a very accurate value because its size determines the angle and path length of armature pivoting these parameters reflect directly on the stroke length over which the print needle is being propelled by the armatures. That path length determines specifically the path traversed by the needle tip to obtain the dot impact printing.
As the needle and armature are set into motion the mass of that moving system may in cases cause the tip of the needle to impact too hard upon the ribbon or the print medium or the platen. Upon return and impacting on the flat abutment rings one may acutally obtain rebounding of the armature and needle. The rebounding effect by and in itself introduces an undesirable delay into the system for the following reasons. The next forward stroke of the needle can begin only (a) after rebounding has stopped, i.e. (b) after the operative magnetic gap has been restored. Hence a delay may occur before satisfactory reenergization obtains and that in turn reduces the print frequency.
Moreover it has to be considered that it is necessary to move the ink ribbon transversely to the print medium so that in general at least two relative motions occur between any needle and that of the needle and head of the ribbon and ink ribbon. In extreme cases the ribbon may actually tear since the needle may still be in too much of a forward position while the ribbon is moved so that the needle may actually tear a hole into the ribbon.
Generally speaking, any rebounding of the needle or of the armature or both cannot be excluded. The resilient material of the flat abutment ring referred to above poses a specific problem as far as rebounding of the individual print needle is concerned. It was found, moreover, that print needles may undertake different stroke length on account of nonuniformity of the flat rings and that in turn deterioriates the appearance of the printed character. Finally it was found that the rebounding generally reduces the use like of the matrix print head.